1. Field of the Invention
The present invention relates to a fuel assembly spacer grid for nuclear reactors, and more particularly to a fuel assembly spacer grid used for a nuclear fuel assembly placed in a reactor core at a nuclear power plant, which spacer grid is provided with swirl deflectors, hydraulic pressure springs, and wear resistant springs.
2. Description of the Prior Art
Referring to FIG. 1, a nuclear reactor is illustrated in which a nuclear fuel assembly is placed in a reactor core denoted by the reference numeral 101. Typically, a spacer grid 103 is used to firmly support fuel elements 111 of the nuclear fuel assembly in a state placed in the reactor core. Such a spacer grid 103 consists of a plurality of longitudinally-extending parallel vertical straps and a plurality of laterally-extending parallel vertical straps perpendicularly interconnecting the longitudinally-extending straps. The fuel elements 111 are placed in internal spaces defined by the interconnecting straps, respectively. The spacer grid 103 serves to prevent the nuclear fuel from being damaged due to vibrations of the fuel elements 111 caused by a flow of cooling water in the reactor core. The spacer grid 103 also maintains a desired space between each fuel element 111 and a guide tube 102 arranged adjacent to the fuel element 111 even when the nuclear reactor is subjected to an earthquake or other external impact. In other words, the spacer grid 103 always provides a flow passage for the cooling water, thereby keeping a desired cooling function for the reactor core. In this regard, active research efforts have been made to provide a spacer grid capable of suppressing vibrations and abrasion of fuel elements while enhancing a resistance to lateral impact.
In order to support the fuel elements 111, the spacer grid 103 has a plurality of protrusions which are typically formed by forming slots at desired portions of the straps, and depressing portions of the straps each positioned between adjacent slots. Of the protrusions, those, which have a low strength, thereby supporting fuel elements while being depressed by those fuel elements, are called "springs". On the other hand, protrusions, which have a high strength, thereby supporting fuel elements while exhibiting little or no deformation, are called "dimples". When springs are subjected to irradiation of neutrons for an extended period of time, they change the property of their material. As a result, the springs exhibit a gradual reduction in elasticity. This results in a reduction in the support force of the springs for the fuel elements, thereby causing those elements to vibrate. Due to such vibrations, the fuel elements may be subjected to a fretting wear at portions contacting the fuel element-supporting elements of the straps. Such a fretting wear of the fuel elements results in a perforation of the fuel elements which, in turn, causes a leakage of radioactivity. In connection with this, several reports have been made.
It is known that an important geometric factor causing the above mentioned fretting wear of fuel elements is the shape of contacts between elements being in contact with each other. In conventional configurations, the contacts between fuel elements and springs or between fuel elements and dimples have a point or line contact shape. In terms of fretting wear, the line contact shape provides a high ability of suppressing vibrations and a high abrasion resistance, as compared to the point contact shape. This is because an increase in contact area at a constant elasticity of springs results in a reduction in the contact pressure causing a depression of those springs, thereby suppressing a fretting wear of fuel elements contacting the springs.
In the case of springs in which elasticity depends only on a material used, as in the above mentioned springs, a reduction in elasticity occurs inevitably due to an irradiation of neutrons onto the springs. In order to eliminate such a drawback, it is necessary to increase the initial spring force of springs. Alternatively, an additional force capable of compensating for the reduced mechanical property of springs should be applied to those springs. However, an increase in the initial spring force may result in an increase in the force required upon initially placing a nuclear fuel assembly, thereby causing a damage of fuel elements.
On the other hand, fuel elements placed in a reactor core exhibit a non-uniform heat flux distribution. Due to such a non-uniform heat flux distribution, a severe increase in the temperature of a cooling water in the reactor core occurs at areas surrounding fuel elements generating a higher heat flux, namely, exhibiting a higher temperature. Meanwhile, bubbles may be locally formed on the surfaces of fuel elements. Where the formation of such bubbles may become severe, thereby covering the surfaces of fuel elements, an abrupt degradation in heat transfer efficiency occurs. This results in an abrupt increase in temperature on the surface of fuel elements. In this case, the temperature of fuel elements themselves or pallets present in the fuel elements may reach a melting point of the fuel elements or pallets. To this end, spacer grids also have a function for forcibly mixing flows of cooling water flowing along areas surrounding fuel elements, thereby obtaining a uniform temperature of the fuel elements while achieving an improvement in the heat transfer performance at the surfaces of the fuel elements. Such a function of spacer grids assists a safe operation of nuclear reactors. For such a function, spacer grids, which include elements for supporting fuel elements, may be attached with separate flow mixing devices adapted to enhance the heat transfer performance.
A typical one of conventional flow mixing methods is a method in which cooling water forms a strong wake when it passes through a spacer grid, thereby mixing flows of the cooling water to promote a temperature uniformity. In such a method, however, the flow mixing function is greatly attenuated as the cooling water flows downstream away from the spacer grid.
Another conventional flow mixing method is a forced swirling method. In accordance with this method, cooling water is swirled in such a manner that cooling water flows of a high density are forced to flow toward the surfaces of fuel elements with bubbles of a low density being concentrated toward the center of swirling. In this case, the layer of the bubbles serves to prevent a reduction in the heat transfer performance, thereby achieving an improvement in the cooling performance of fuel elements. It is known that the forced swirling method exhibits a slow attenuation in flow mixing effect generated when cooling water passes through the spacer grid, as compared to the wake forming method. Recent developments of spacer grids are focused on the formation of swirling flows.
The loss of pressure in a cooling water flow generated when the cooling water passes an obstacle depends mainly upon an area projected onto a plane normal to the flow direction of the cooling water. The provision of a flow mixer results in an additional pressure loss because an increase in the projected area causes a reduction in the area through which the cooling water flows.
Such an increase in pressure loss results in an increase in the load applied to a pump for pumping the cooling water. For this reason, there is a problem in that the flow rate of the cooling water flowing in the nuclear reactor decreases. Therefore, where a flow mixer is attached to the spacer grid, a design capable of minimizing the loss of pressure at the same projected area should be provided.
Recent developments of nuclear fuel are concentrated on a highly burn-up and non-defective fuel. In the case of a highly-combustible fuel, an increase in the nuclear fuel concentration may be involved. In this case, a severe heat flux peaking phenomenon may occur. Here, the output peaking phenomenon is a phenomenon wherein a part of fuel elements generate a heat flux considerably higher than the mean heat flux of those fuel elements. Where such a severe heat flux peaking phenomenon occurs, a severe boiling phenomenon occurs on the surfaces of fuel elements. This results in a high possibility of a great degradation in heat transfer rate. To this end, it is required to develop a spacer grid with a superior cooling performance over conventional spacer grids. Due to a high burn-up capacity, the using period of the nuclear fuel is extended. In this case, the amount of neutrons irradiated onto the spacer grid increases. This is an important consideration in that the problem associated with a decrease in spring force may occur due to a change in the property of the spacer grid material. For the development of a non-defective fuel, therefore, it is necessary, in terms of fretting wear, to provide a mechanism capable of compensating for a decrease in the spring force required for suppressing vibrations of fuel elements.